1. Field of the Invention
The present invention relates to a control system for controlling the rotational speeds of wheels in automotive vehicles and, more particularly, to the control system such as, for example, anti-lock controllers and traction controllers designed to control braking forces or driving forces applied to wheels that avoid excessive increases in the absolute value of the difference between the wheel rotational speeds and the vehicle running speed, or the appropriately inferred or representative value thereof.
2. Background Information
The prior art wheel speed control system, which is employed, for example, in an anti-lock controller or a traction controller, generally makes two decisions: a decision to determine if the absolute value of the degree of acceleration of the wheel rotational speeds has exceeded a certain limit; and a decision to determine if the absolute value of the difference between the wheel rotational speeds and the vehicle running speed has exceeded a certain limit.
According to the prior art wheel speed control system, when a slightly excessive braking force or a slightly excessive driving force is applied continuously while driving on a road, the frictional force between the wheel tires and the road surface is excessively small, such as when driving on a slippery road due to the presence of, for example, ice. During this time, the absolute value of the difference between the wheel rotational speeds and the vehicle running speed increases only moderately, resulting in a delay of the detection of slip or skid.
More specifically, where the absolute value of the wheel acceleration is zero or very small, the slip or skid cannot be detected unless the absolute value of the difference exceeds a threshold value. When noise components contained in the wheel rotational speeds are taken into consideration, the above-mentioned threshold value cannot be reduced.
Although this problem would bring about relatively little harm in practice in the case of the anti-lock controller because the slightly excessive braking force will not be applied continuously for a prolonged time, this problem cannot be neglected in the case of the traction controller.